This study is directed toward understanding the biochemistry, action and production of a subclass of the colony-stimulating factor (CSF) that stimulates mononuclear phagocyte proliferation (CSF-1). CSF-1 modulation of the CSF-1 receptor and CSF-1-induced morphological changes have been studied in bone marrow-derived macrophages (BMM). CSF-1 has been shown to both stimulate the rate of BMM protein synthesis and inhibit the rate of BMM protein degradation. The CSF-1 receptor has been solubilized and characterized in detergent solution.